The work proposed is an investigation of certain aminoacyl tRNA synthetases in developing red blood cells. Synthetases for histidyl and methionyl tRNA will be studied initially because of special involvement of these tRNA species in hemoglobin synthesis. The investigations will develop information concerning the abundance and specificity of these synthetases that can be related to the roles of their cognate tRNA species in hemoglobin synthesis. If possible, evidence will be developed for multiple forms of these enzymes (isoenzymes) as have been noted for synthetases for other aminoacyl tRNA's in animal cells. Approaches will include the extraction, separation, and purification of these enzymes from rabbit reticulocytes by well established techniques. These procedures will be followed by the determination of kinetic, physical, and chemical properties of the enzymes, with particular interest in establishing the nature of enzyme multiplicity when it is observed. The distribution of multiple species will be determined both within various cellular fractions (especially mitochondria) and among different cell types. The specificity of multiple forms for tRNA iso-accepting species will be investigated.